Conventionally, as a method of preparing magnetic substance-containing polymer particles, there are known (1) a method of making the prepared polymer particle contain an iron ion and producing the magnetic substance, (2) a method of adding the prepared magnetic substance particle to the polymer particle in the step of polymerizing monomers (refer to Japanese Kokai Publication Hei-9-208788) and (3) a method of combining the polymer particle and the magnetic substance particle, which are separately prepared (refer to Japanese Kokai Publication Hei-6-231957). In addition to these, there is (4) a method of coating the magnetic substance particle with a polymer and the like (refer to Japanese Kokai Publication Hei-6-92640).
In the method (1), there was an issue that since the iron ion was absorbed in the polymer particle, the magnetic substance was exposed at the surface and the magnetic substance was oxidized. And, in the method (2), there was an issue that the magnetic substance particle was not uniformly taken in into the polymer particle, or an issue that the polymer particles have a wide particle size distribution since control of a particle size is difficult. And, in the method (3), there was an issue that since the polymer particles flocculate, this method cannot be applied to the case of small particle sizes. Further, in the method (4), since it is impossible to be coated uniformly, a suspending property and dispersibility are poor, and a part of the surface of a magnetic substance particle may be exposed.
On the other hand, as a method of immunoassay in a trace amount, radioimmunoassay, enzyme immunoassay and fluorescence immunoassay have been previously known and already put into practical use. Each of these methods is a method in which the antigen or the antibody, to which an isotope, an enzyme or a fluorescent material is added as a marker, is used and the presence of an antibody or an antigen, which specifically reacts with the antigen with a marker or the antibody with a marker, is detected. On the occasion of such a method of immunoassay, a magnetic substance-encapsulated particle is used in order to carry out the B/F separation efficiently and easily. And, a use other than the B/F separation (refer to Japanese Kokai Publication 2000-88852), and a method of immunoassay using the magnetic substance-encapsulated particle itself as a marker (Japanese Kokai Publication Hei-6-148189, Japanese Kokai Publication Hei-7-225233, Japanese kohyo Publication 2001-524675) are disclosed.
In a method of immunoassay in which the magnetic substance-encapsulated particle itself is used as a marker, a measuring precision thereof depends on the homogeneity of the magnetic substance-encapsulated particle, that is, the uniformity of the content of the magnetic substance in every particle. However, commercially available magnetic substance-encapsulated particles have variations in the content of the magnetic substance, and it is difficult to control the uniformity of the content of the magnetic substance by publicly known method of producing the magnetic substance-encapsulated particles.
When the magnetic substance-encapsulated particles are used for a method of immunoassay, there may be cases of handling the magnetic substance-encapsulated particles in the form of a dispersion liquid dispersed in a buffer solution and the like in a step of binding an antigen or an antibody to it or a step of mixing it with a substance to be detected. Therefore, it is desirable that the magnetic substance-encapsulated particle has such high dispersion stability that particles do not precipitate naturally in a state of a dispersion liquid. However, commercially available magnetic substance-encapsulated particles have a problem of handling that a part of the particles precipitate when they are left standing in a state of a dispersion liquid for a while, and the like.